Document reservation of double underscore names (dunders) (#51215)

In the recent discussion about `main`, `__main__` was proposed. One
reason in favor of this is that double underscore, or dunder, names are
reserved names in Julia. This however is not documented.

This pull request adds a paragraph reserving double underscore names for
Julia.

They are added to Base -> Essentials -> Keywords where other reserved
names are documented.

https://docs.julialang.org/en/v1.11-dev/base/base/#Keywords

cc: @StefanKarpinski

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Co-authored-by: 3f6a <126961207+3f6a@users.noreply.github.com>